Scientifically we do not; and for a man of science to pretend, or to assert in a popular treatise, that we do, is essentially and seriously to mislead. (See Chapter VII. below.) It may even be a question whether the assertion of entire ignorance at the present time is completely appropriate, whether we have not some positive evidence _against_ Professor Haeckel's contention. I believe that we have; and though I may acquiesce in an assertion of present ignorance, I am not at all willing to accept the next sentence of Professor du Bois-Reymond's answer, and to say _ignorabimus_, we never shall know. The matter seems to me within the legitimate lines of scientific inquiry, and it is unwise to attempt prediction, especially negative prediction, or to attempt to close the door to the future developments of knowledge. But I am content to say for the present that from the point of view of strict science it is not yet possible to give any positive answer to these questions; that they must await the progress of discovery. It becomes a question of some interest, therefore, how it is possible for Professor Haeckel and for others of his school to have arrived at the idea not only that a scientific answer can be given, but that already it has been given, and that they know distinctly what it is. Note on the Word "Life." Until a term is accurately defined, and even afterwards for some purposes, it is permissible to use a word of large significance in more than one sense. Thus the word "light" may be considered a psychological term, denoting a certain sensation, or a physiological term, signifying the stimulus of certain specialised nerve-endings, or a physical term, expressing briefly an electromagnetic wave-disturbance in the ether. I am using the word "life" in a quite general sense, as is obvious, for if it be limited to certain metabolic processes in protoplasm—which is the narrowest of its legitimate meanings—what I have said about its possible existence apart from matter would be absurd. It may be convenient to employ the word "vitality" for this limited sense; but so far as I know, there is no general consensus of usage, and the context must suffice to show a friendly reader the connotation intended. CHAPTER III THE DEVELOPMENT OF LIFE This leads me to the second main thesis or central scientific doctrine of Professor Haeckel's treatise, the biological one; and it is this which I shall now proceed to illustrate by further quotations, viz., the connection as he conceives it between life and matter.